Biomedical Engineering Reference
In-Depth Information
and it is possible to verify less bone resorption for half coated stems and also the
hypertrophy near stem tip due to distal peak on contact stress.
This concurrent model for remodeling and ingrowth confirmed the good perfor-
mance of optimized shapes. Some nondominated points have more bone ingrowth
and less bone resorption than initial shape. Additionally, all nondominated solu-
tions promote less bone loss in the proximal outside region of femur (zone E)
compared to the initial shape. Notice, that bone resorption in this particular region
is one of the major reasons for THA failure [33].
10.8
Concluding Remarks
A new multicriteria shape optimization procedure has been developed in order to
obtain cementless hip prostheses with improved performance. The multicriteria
objective function combines three major causes of implant failure, the relative
tangential displacements, normal contact stress, and stress shielding effect.
A concurrent model for bone remodeling and osseointegration was also used
to study the long-term performance of initial shape and optimized geometries.
The remodeling process used was developed by Fernandes
et al
. [45] and assumes
that bone adapts to the mechanical loads in order to maximize the stiffness and
minimize the metabolic cost to maintain bone tissue. An algorithm for bone
ingrowth is proposed, taking into account the five interfacial conditions that must
be satisfied to obtain osseointegration into the porous coating.
Stem shapes obtained with the three single cost functions are contradictory.
Nevertheless, several geometric characteristics obtained are in accordance with
clinical observations. The minimization of displacement function
f
d
leads to
stems with wedge design to improve axial stability, and some sections are almost
rectangular and the elbow is more accentuated to improve rotational stability
[50, 51]. With the minimization of contact stress function
f
t
stem tip is small and
circular to avoid direct contact with the cortical bone [52] and proximal sections
are thicker to reduce stress at calcar region. The minimization of remodeling
objective function
f
r
leads to minimally invasive stems without elbow [33]. The
multicriteria optimization is highly efficient in dealing with these differences, since
nondominated solutions obtained have a combination of geometric characteristics
depending on weight coefficients:
r
. In fact, from a computational
point of view, nondominate solutions have a better performance than the initial
prosthesis, that is, the three single functions are simultaneously reduced.
Results obtained with the model for remodeling and bone ingrowth illustrates
the dependence of ingrowth on bone remodeling and vice versa. This model
confirms the importance of stem stability in long-term success of uncemented hip
implants. The long-term prediction obtained with this model is also in agreement
with clinical observations. For instance, Niinim
β
d
,
β
t
,and
β
aki
et al
. [33] and Sluimer
et al
. [34]
also verified that totally coated stems are more stable but partially coated stems lead
to less bone resorption. In addition, minimally invasive stems have less stability
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